Method and apparatus for generating non-linear frequency modulation signal in real time and computer storage medium

ABSTRACT

A method and apparatus for generating a NLFM signal in real time, and a computer storage medium are disclosed, including: determining a signal parameter of a signal according to a system parameter, the signal parameter includes: a signal bandwidth, a signal pulse width and a PSLR; determining a power spectrum density function according to PSLR; calculating the power spectrum density function to obtain a group delay vector; calculating a frequency axial vector according to a system sampling rate; calculating a time axial vector according to the signal pulse width; performing linear interpolation calculation on the group delay vector to obtain an instantaneous frequency vector; integrating the instantaneous frequency vector to obtain a phase vector; determining a signal time domain discrete vector; and generating a digital signal according to the signal time domain discrete vector, and performing digital-to-analog conversion on the digital signal to obtain the NLFM signal.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims benefit of Chinese Patent Application No.201910045019.0, filed on Jan. 17, 2019, the contents of which are herebyincorporated by reference in their entirety.

TECHNICAL FIELD

The disclosure relates to the technical field of signal processing, andin particular to a method and an apparatus for generating a Non-LinearFrequency Modulation (NLFM) signal in real time, and a computer storagemedium.

BACKGROUND

A Synthetic Aperture Radar (SAR) can observe the earth in allday-and-night, all weathers and all over the globe, and is appliedwidely. At present, a Linear Frequency Modulation (LFM) signal have atransmitting waveform that is most commonly used in an SAR system. For aresponse function generated by such a waveform via matched filtering, aPeak Side Lobe Ratio (PSLR) after normalization is −13 decibels (dBs).In order to suppress a height of a side lobe, a weighted windowfunction, an adaptive filtering algorithm and an optimization algorithmare often used. However, these methods will make a matched filterunmatched and reduce the Signal to Noise Ratio (SNR) of the output.

Compared with the LFM signal, a very small PSLR may be obtained by apulse compression of an NLFM signal, and the SNR of the output is notreduced in this process. From relevant experiments, the NLFM signal mayreduce a loss of the SNR by 1-2 dB, which means that 25% of transmittedpower of an antenna is saved. For the energy starved radar system, thesystem performance may be improved by using the NLFM signal as thetransmitting waveform.

Presently, a common technical method for producing the NLFM signalincludes the following operations: a discrete value of the signal isobtained by a calculation process at ground, and then stored in aRead-Only Memory (ROM) of a Field-Programmable Gate Array (FPGA). Inuse, discrete data stored in the ROM is read, and subjected todigital-to-analog conversion to generate the NLFM signal. For such amethod, the signal needs to be collected and stored first and theninvoked and converted to generate the NLFM signal. The method isrestricted by a limited storage space of the ROM. Due to the storagemode, the NLFM signal can only be used in a scientific experiment, andcannot be promoted in engineering.

Therefore, how to decrease time for generating the NLFM signal so as togenerate the NLFM signal in real-time is a problem to be solved urgentlyat present.

SUMMARY

The embodiments of the disclosure provide a method and an apparatus forgenerating an NLFM signal in real time.

The embodiments of the disclosure provide a method for generating anNLFM signal in real time, which includes the following operations.

A signal parameter of a signal is determined according to a systemparameter, the signal parameter at least includes: a signal bandwidth, asignal pulse width and a PSLR.

A power spectrum density function is determined according to the PSLR.

The power spectrum density function is calculated to obtain a groupdelay vector.

A frequency axial vector is calculated according to a system samplingrate.

A time axial vector is calculated according to the signal pulse width.

Linear interpolation calculation is performed on the group delay vectorby using the frequency axial vector and the time axial vector, to obtainan instantaneous frequency vector.

The instantaneous frequency vector is integrated to obtain a phasevector.

A signal time domain discrete vector is determined according to thephase vector.

A digital signal is generated according to the signal time domaindiscrete vector, and digital-to-analog conversion is performed on thedigital signal to obtain the NLFM signal.

In the above-mentioned solutions, the operation that a power spectrumdensity function is determined according to the PSLR includes thefollowing operation.

A window function corresponding to the PSLR is obtained according to thePSLR, and the power spectrum density function is determined according tothe window function.

In the above-mentioned solutions, the operation that the power spectrumdensity function is calculated to obtain a group delay vector includesthe following operation.

Discrete integration is performed on the power spectrum density functionto obtain the group delay vector.

In the above-mentioned solutions, the operation that linearinterpolation calculation is performed on the group delay vector byusing the frequency axial vector and the time axial vector to obtain aninstantaneous frequency vector includes the following operations.

The group delay vector is divided into n group delay subvectors, n is apositive integer greater than 1.

The linear interpolation calculation is performed on the n group delaysubvectors respectively by using the frequency axial vector and the timeaxial vector to obtain n instantaneous frequency vectors.

The operation that the instantaneous frequency vector is integrated toobtain a phase vector includes the following operation.

The n instantaneous frequency vectors are respectively integrated, andintegrated results are spliced to obtain the phase vector.

The embodiments of the disclosure provide an apparatus for generating anNLFM signal in real time, which includes: a processor; a memory forstoring instructions; and a digital to analog converter, wherein theprocessor is configured to execute the instructions to: determine asignal parameter of a signal according to a system parameter, the signalparameter at least includes: a signal bandwidth, a signal pulse widthand a PSLR; determine a power spectrum density function according to thePSLR; calculate the power spectrum density function to obtain a groupdelay vector; calculate a frequency axial vector according to a systemsampling rate; calculate a time axial vector according to the signalpulse width; perform linear interpolation calculation on the group delayvector by using the frequency axial vector and the time axial vector toobtain an instantaneous frequency vector; integrate the instantaneousfrequency vector to obtain a phase vector; determine a signal timedomain discrete vector according to the phase vector; and generate adigital signal according to the signal time domain discrete vector, andperform digital-to-analog conversion on the digital signal to obtain theNLFM signal via the digital to analog converter.

In the above-mentioned solutions, the processor is further configured toexecute the instructions to: obtain a window function corresponding tothe PSLR according to the PSLR, and determine the power spectrum densityfunction according to the window function.

In the above-mentioned solutions, the processor is further configured toexecute the instructions to: perform discrete integration on the powerspectrum density function to obtain the group delay vector.

In the above-mentioned solutions, the processor is further configured toexecute the instructions to divide the group delay vector into n groupdelay subvectors, n is a positive integer greater than 1; and perform,by using the frequency axial vector and the time axial vector, thelinear interpolation calculation on the n group delay subvectorsrespectively, to obtain n instantaneous frequency vectors.

The processor is further configured to execute the instructions tointegrate the n instantaneous frequency vectors respectively, and spliceintegrated results to obtain the phase vector.

The embodiments of the disclosure provide a computer readable storagemedium having computer programs stored thereon; and the computerprograms implement, when being executed by a processor, any operation ofthe method for generating the NLFM signal in real time in theabove-mentioned solutions.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings generally show the embodiments discussed inthe specification in an illustrative manner rather than a restrictivemanner.

FIG. 1 schematically illustrates a flowchart of a method for generatingan NLFM signal in real time according to an embodiment of thedisclosure.

FIG. 2 schematically illustrates a structural diagram of an apparatusfor generating an NLFM signal in real time according to an embodiment ofthe disclosure.

FIG. 3 schematically illustrates a flowchart of a method for generatinga time domain signal according to an embodiment of the disclosure.

FIG. 4 schematically illustrates a plane effect of a linearinterpolation algorithm according to an embodiment of the disclosure.

FIG. 5 schematically illustrates a structural diagram of an apparatusimplemented based on FPGA hardware according to an embodiment of thedisclosure.

FIG. 6 schematically illustrates a time sequence for generating an NLFMsignal based on an SAR system according to an embodiment of thedisclosure.

FIG. 7 is an image of a real part of a time domain signal of an analogsignal result collected by an oscilloscope according to an embodiment ofthe disclosure.

FIG. 8 is an image of a real part of a time domain signal after pulsecompression according to an embodiment of the disclosure.

DETAILED DESCRIPTION

In order to understand the characteristics and technical contents in theembodiments of the disclosure more specifically, the implementation foreach embodiment of the disclosure will be described below in detail incombination with the accompanying drawings. The accompanying drawingsare merely for reference, rather than a limit for the embodiments of thedisclosure.

The embodiments of the disclosure provide a method for generating anNLFM signal in real time. As shown in FIG. 1 , the method includes thefollowing operations.

At Operation 101: a signal parameter of a signal is determined accordingto a system parameter, the signal parameter at least includes: a signalbandwidth, a signal pulse width and a PSLR. The system is a radarsystem, and the signal is a signal transmitted by the system.

In some embodiments, the parameter of the radar system is a parameter ofan SAR system.

In some embodiments, the signal is a radar transmitting signal.

Specifically, the system parameter refers to a parameter related to thesignal in the SAR system, the system parameter may include an SNR and arange resolution. The SNR is an SNR output by the radar system, and isused for determining a pulse width of the signal, that is, determiningthe signal pulse width. The range resolution refers to a minimum spacefor distinguishing two targets at different distances, and is used fordetermining the signal bandwidth.

In the embodiments of the present disclosure, the B denotes the signalbandwidth, and the T_(p) denotes the signal pulse width.

At Operation 102: a power spectrum density function is determinedaccording to the PSLR.

In some embodiments, the operation 102 includes the followingoperations: a window function corresponding to the PSLR is obtainedaccording to the PSLR, and the power spectrum density function isdetermined according to the window function.

Herein, the window function corresponding to the PSLR is obtainedaccording to the PSLR based on correspondence between PSLRs and windowfunctions, which is set in advance.

In some embodiments, the correspondence between PSLRs and windowfunctions may be set in a correspondence table. When there is a need toobtain a window function, a required PSLR is used as an index, and awindow function corresponding to a PSLR having a value same as therequired PSLR is found in the correspondence table.

The window function may include: a Hamming window, a Kaiser window and aTaylor window.

At Operation 103: the power spectrum density function is calculated toobtain a group delay vector.

In some embodiments, the operation 103 includes an operation thatdiscrete integration is performed on the power spectrum density functionto obtain the group delay vector.

Specifically, the group delay may be calculated according to the powerspectrum density function by a formula (1):

$\begin{matrix}{{G(f)} = {\int_{0}^{f}{\frac{P(f)}{C}df}}} & (1)\end{matrix}$

Where, the f denotes a frequency, the P(f) denotes a power spectrumdensity function with the frequency as an independent variable, and theC is an inverse proportional coefficient, and may be denoted by aformula (2):

$\begin{matrix}{C = {\frac{1}{T_{p}}{\int_{B/2}^{{- B}/2}{{P(f)}df}}}} & (2)\end{matrix}$

During hardware implementation, discrete integration may be used and thegroup delay may be denoted as a group delay vector {right arrow over(g)}, where, the {right arrow over (g)} may be denoted by a formula (3):{right arrow over (g)}=(g ₀ ,g ₁ , . . . ,g _(i))  (3)Where, 0≤i≤N−1, the g_(i) denotes a discrete value of G(f) and the N isthe number of sampling points; and N=└F_(s)T_(p)┘, where the └⋅┘ denotesto round down.

In some embodiments, a Taylor window function may be selected as thepower spectrum density function, and a corresponding group delayfunction G(f) may be denoted by a formula (4):

$\begin{matrix}{{G(f)} = {{\frac{T_{p}}{B}f} + {\sum\limits_{m = 1}^{\overset{\_}{m} - 1}{\frac{F_{m}T_{p}}{\pi m}{\sin\left( {\frac{2\pi m}{B}f} \right)}}}}} & (4)\end{matrix}$Where, the F_(m) is a Taylor window coefficient, the m is the number ofside lobes having a same height, and the m is a calculation processquantity.

At Operation 104: a frequency axial vector is calculated according to asystem sampling rate.

The frequency axial vector {right arrow over (f)} is calculatedaccording to the system sampling rate F_(s), wherein the {right arrowover (f)} may be denoted by a formula (5):{right arrow over (f)}[f ₀ ,f ₁ , . . . ,f _(i)]  (5)

-   -   Where

${0 \leq i \leq {N - 1}},{f_{i} = {{- \frac{F_{s}}{2}} + {i \times {\frac{F_{s}}{N}.}}}}$

The system sampling rate F_(s) is a default value set when the system isdesigned. In some embodiments, the system sampling rate F_(s) may bedetermined based on the system parameter.

At Operation 105: a time axial vector is calculated according to thesignal pulse width.

The time axial vector {right arrow over (t)} may be denoted by a formula(6):{right arrow over (t)}=[t ₀ ,t ₁ , . . . ,t _(i)]  (6)

-   -   Where,

${0 \leq i \leq {N - 1}},{t_{i} = {{- \frac{T_{p}}{2}} + {i \times {\frac{1}{F_{s}}.}}}}$

At Operation 106: linear interpolation calculation is performed on thegroup delay vector by using the frequency axial vector and the timeaxial vector to obtain an instantaneous frequency vector.

Linear interpolation is performed on the group delay vector {right arrowover (g)} according to the frequency axial vector {right arrow over(f)}, the time axial vector {right arrow over (t)}, to calculate theinstantaneous frequency vector {right arrow over (F)}, wherein the{right arrow over (F)} may be denoted by a formula (7):{right arrow over (F)}=(f _(t) ₀ ,f _(t) ₁ , . . . ,f _(t) _(i))=interp1[{right arrow over (g)},{right arrow over (f)},{right arrowover (t)}]  (7)Where, the interp1 function is a one-dimensional linear interpolationfunction; and a correspondence between horizontal and verticalcoordinates is determined based on ({right arrow over (f)}, {right arrowover (g)}) within a two-dimensional coordinate plane, a verticalcoordinate corresponding to a horizontal coordinate {right arrow over(t)} calculated via linear interpolation is:{right arrow over (F)}=interp1[{right arrow over (g)},{right arrow over(f)},{right arrow over (t)}]

Further, the vertical coordinate f_(t), corresponding to the horizontalcoordinate t_(i) may be denoted by a formula (8):

$\begin{matrix}{f_{t_{i}} = {\frac{g_{p} - g_{q}}{f_{p} - f_{q}} \times t_{i}}} & (8)\end{matrix}$The t_(i) falls into an interval [g_(p), g_(p)], 0≤q≤i≤p≤N−1, and thef_(p) and the f_(q) are vertical coordinates corresponding to horizontalcoordinate g_(p) and g_(q), respectively.

At Operation 107: the instantaneous frequency vector is integrated toobtain a phase vector.

The signal phase vector {right arrow over (ϕ)} is obtained byintegrating according to the instantaneous frequency vector, and the{right arrow over (ϕ)} may be denoted by a formula (9):{right arrow over (ϕ)}=[ϕ₀,ϕ₁, . . . ,ϕ_(i)]  (9)

-   -   Where, when i=0, ϕ₀=0.    -   When 1≤i≤N−1,

$\phi_{i} = {{{\pi\left( {f_{t_{i}} + f_{t_{i - 1}}} \right)} \times \frac{1}{F_{s}}} + {\phi_{i - 1}.}}$

In some embodiments, the operation 106 includes the followingoperations.

The group delay vector {right arrow over (g)} is divided into n groupdelay subvectors, the n is a positive integer greater than 1.

The linear interpolation calculation is respectively performed on the ngroup delay subvectors by using the frequency axial vector {right arrowover (f)} and the time axial vector {right arrow over (t)} to obtain ninstantaneous frequency vectors.

The operation 107 includes the following operation.

The n instantaneous frequency vectors are respectively integrated, andintegrated results are spliced to obtain the phase vector {right arrowover (ϕ)}.

In some embodiments, with n=2 as an example: the signal phase vectors,i.e. {right arrow over (ϕ)}₁=[ϕ₀, ϕ₁, . . . , ϕ_(h)] and {right arrowover (ϕ)}₂=[ϕ_(h+1), ϕ_(h+2), . . . , ϕ_(i)] are calculated by using twoinstantaneous frequency vectors obtained in the operation 106, where0≤h≤i≤N−1. Supposing that ϕ₀=0 and ϕ_(h+1)=0 are set in advance, the{right arrow over (ϕ)}₁ and the {right arrow over (ϕ)}₂ may berespectively calculated as per the operation 107. At last, a final valueof the {right arrow over (ϕ)}₁ serves as an initialized value {rightarrow over (ϕ)}₂=[ϕ_(h+1), ϕ_(h+2), . . . , ϕ_(i)]+ϕ_(h) of the {rightarrow over (ϕ)}₂; and as a consequence, the {right arrow over (ϕ)}₁ andthe {right arrow over (ϕ)}₂ are directly spliced into an integral phasevector {right arrow over (ϕ)}.

At Operation 108: a signal time domain discrete vector is determinedaccording to the phase vector.

Specifically, the signal time domain discrete vector {right arrow over(s)} may be denoted by a formula (9):s=[s ₀ ,s ₁ , . . . ,s _(i)]  (10)

-   -   Where, 0≤i≤N−1, S_(i)=exp(j2πϕ_(i)).

At Operation 109: a digital signal is generated according to the signaltime domain discrete vector, and digital-to-analog conversion isperformed on the digital signal to obtain the NLFM signal.

The digital-to-analog conversion means that a discrete digital quantityis converted into an analog quantity that is changed continuously. Insome embodiments, the digital-to-analog conversion may be performed viaa Digital to Analog Converter (DAC).

Generating in real-time means generating within a limited time, and therange of the limited time is determined according to an actual demand.In this embodiment of the disclosure, for the real-time generation ofthe NLFM signal, the limited time range is determined with a computerprocessing speed and a communication transmitting speed of the radarsystem as a standard, and generally reaches to a microsecond.

The embodiments of the disclosure provide an apparatus for generating anNLFM signal in real time. As shown in FIG. 2 , the apparatus includes:an initialization module 301, a power spectrum density functiondetermination module 302, a group delay vector calculation module 303, afrequency axial vector calculation module 304, a time axial vectorcalculation module 305, an instantaneous frequency vector calculationmodule 306, a phase vector calculation module 307, a discrete vectorcalculation module 308, and a conversion module 309.

The initialization module 301 is configured to determine a signalparameter of a signal according to a system parameter, the signalparameter at least includes: a signal bandwidth, a signal pulse widthand a PSLR.

In the embodiments of the disclosure, the B denotes the signalbandwidth, and the T_(p) denotes the signal pulse width.

The power spectrum density function determination module 302 isconfigured to determine a power spectrum density function according tothe PSLR.

In some embodiments, the power spectrum density function determinationmodule 302 is specifically configured to: obtain a window functioncorresponding to the PSLR according to the PSLR, and determine the powerspectrum density function according to the window function.

Herein, the window function corresponding to the PSLR may be obtainedaccording to the PSLR based on correspondence between PSLRs and windowfunctions, which is set in advance.

In some embodiments, the correspondence between PSLRs and windowfunctions may be set in a correspondence table. When there is a need toobtain the window function, a required PSLR is used as an index, and thewindow function corresponding to the PSLR having a value same as therequired PSLR is found in the corresponding table.

The window function may include: a Hamming window, a Kaiser window and aTaylor window.

The group delay vector calculation module 303 is configured to calculatethe power spectrum density function to obtain a group delay vector.

In some embodiments, the group delay vector calculation module 303 isspecifically configured to: perform discrete integration on the powerspectrum density function to obtain the group delay vector.

Specifically, the group delay may be calculated according to the powerspectrum density function as per a formula (1).

During hardware implementation, discrete integration may be used and thegroup delay may be denoted as a group delay vector {right arrow over(g)}, where, the {right arrow over (g)} may be denoted by a formula (3).

In some embodiments, a Taylor window function may be selected as thepower spectrum density function, and a corresponding group delayfunction G(f) may be denoted by a formula (4).

The frequency axial vector calculation module 304 is configured tocalculate a frequency axial vector according to a system sampling rate.

In some embodiments, the frequency axial vector calculation module 304is specifically configured to: calculate the frequency axial vector{right arrow over (f)} according to the system sampling rate F_(s),wherein the {right arrow over (f)} may be denoted by a formula (5).

The time axial vector calculation module 305 is configured to calculatea time axial vector according to the signal pulse width.

In some embodiments, the time axial vector {right arrow over (t)} may bedenoted by a formula (6).

The instantaneous frequency vector calculation module 306 is configuredto perform linear interpolation calculation on the group delay vector byusing the frequency axial vector and the time axial vector to obtain aninstantaneous frequency vector.

In some embodiments, the instantaneous frequency vector calculationmodule 306 is specifically configured to: perform linear interpolationto the group delay vector {right arrow over (g)} according to thefrequency axial vector {right arrow over (f)}, the time axial vector{right arrow over (t)}, to calculate the instantaneous frequency vector{right arrow over (F)}, wherein the instantaneous frequency vector maybe denoted by a formula (7).

The interp1 function is a one-dimensional linear interpolation function;and a correspondence between horizontal and vertical coordinates isdetermined based on ({right arrow over (g)}, {right arrow over (f)})within a two-dimensional coordinate plane, a vertical coordinatecorresponding to a horizontal coordinate {right arrow over (t)} iscalculated via linear interpolation as {right arrow over(F)}=interp1[{right arrow over (g)}, {right arrow over (f)}, {rightarrow over (t)}]

Further, the corresponding vertical coordinate f_(t), corresponding tothe horizontal coordinate t_(i) may be denoted by a formula (8).

The phase vector calculation module 307 is configured to integrate theinstantaneous frequency vector to obtain a phase vector.

In some embodiments, the phase vector calculation module 307 isspecifically configured to: perform integrating according to theinstantaneous frequency vector to obtain the signal phase vector {rightarrow over (ϕ)}, and the {right arrow over (ϕ)} may be denoted by aformula (9).

In some embodiments, the instantaneous frequency vector calculationmodule 306 is further configured to: divide the group delay vector{right arrow over (g)} into n group delay subvectors, n is a positiveinteger greater than 1; and respectively perform the linearinterpolation calculation on the n group delay subvectors by using thefrequency axial vector {right arrow over (f)} and the time axial vector{right arrow over (t)} to obtain n instantaneous frequency vectors.

The phase vector calculation module 307 is further configured to:integrate the n instantaneous frequency vectors respectively, and spliceintegrated results to obtain the phase vector {right arrow over (ϕ)}.

The discrete vector calculation module 308 is configured to determine asignal time domain discrete vector according to the phase vector.Specifically, the signal time domain discrete vector {right arrow over(s)} may be denoted by a formula (9).

The conversion module 309 is configured to generate a digital signalaccording to the signal time domain discrete vector, and performdigital-to-analog conversion on the digital signal to obtain the NLFMsignal.

The embodiments of the disclosure provide a method for generating a timedomain signal, which may be applied to hardware implementation. As shownin FIG. 3 , the method includes the following operations.

At Operation 401: a signal parameter is initialized as required.

In this embodiment of the disclosure, the operation 401 that a signalparameter is initialized as required includes an operation that arequired signal parameter is determined according to a system parametersuch as an SNR and a range resolution, the signal parameter includes asignal bandwidth B, a signal pulse width T_(p) and a PSLR; and a powerspectrum density function is determined according to the PSLR.

At Operation 402: a group delay vector is calculated according to apower spectrum density function.

Specifically, the group delay may be calculated according to the powerspectrum density function determined in the operation 401 as per afollowing formula.

${G(f)} = {\int_{0}^{f}{\frac{P(f)}{C}df}}$Where, the P(f) denotes a power spectrum density function. In someembodiments, a window function (for example, a Hamming window, a Kaiserwindow, a Taylor window etc.) may be used as the power spectrum densityfunction. The C is an inverse proportional coefficient,

${C = {\frac{1}{T_{p}}{\int_{B/2}^{{- B}/2}{{P(f)}df}}}}.$

In some embodiments, during hardware implementation, discreteintegration may be used and the group delay may be denoted as a groupdelay vector {right arrow over (g)}=(g₀, g₁, . . . , g_(i)), where,0≤i≤N−1, the g_(i) denotes a discrete value of G(f) and the N is thenumber of sampling points; and N=└F_(s)T_(p)┘, where the └⋅┘ denotes toround down.

In some embodiments, a Taylor window function may be selected as thepower spectrum density function, and a corresponding group delayfunction may be denoted as

${G(f)} = {{\frac{T_{p}}{B}f} + {\sum\limits_{m = 1}^{\overset{\_}{m} - 1}{\frac{F_{m}T_{p}}{\pi m}{{\sin\left( {\frac{2\pi m}{B}f} \right)}.}}}}$Where, the F_(m) is a Taylor window coefficient, the m is the number ofside lobes having a same height, and the m is a calculation processquantity.

At Operation 403: linear interpolation is performed according to thegroup delay vector to determine an instantaneous frequency vector.

Specifically, a frequency axial vector {right arrow over (f)} calculatedaccording to the signal pulse width, the signal bandwidth and the systemsampling rate F_(s) is{right arrow over (f)}=[f ₀ ,f ₁ , . . . ,f _(i)]

-   -   Where, 0≤i≤N−1,

${f_{i} = {{- \frac{F_{s}}{2}} + {i \times \frac{F_{s}}{N}}}}.$

A time axial vector {right arrow over (t)} is{right arrow over (t)}=[t ₀ ,t ₁ , . . . ,t _(i)]

-   -   Where, 0≤i≤N−1,

${t_{i} = {{- \frac{T_{p}}{2}} + {i \times \frac{1}{F_{s}}}}}.$

The linear interpolation is performed on the group delay vector {rightarrow over (g)} according to the frequency axial vector {right arrowover (f)}, the time axial vector {right arrow over (t)}, to calculatethe instantaneous frequency vector {right arrow over (F)} as{right arrow over (F)}=(f _(t) ₀ ,f _(t) ₁ , . . . ,f _(t) _(i))=interp1[{right arrow over (g)},{right arrow over (f)},{right arrowover (t)}]

The interp1[{right arrow over (g)}, {right arrow over (f)}, {right arrowover (t)}] denotes a one-dimensional linear interpolation operator.Particularly, according to correspondence between horizontal andvertical coordinates is determined according to ({right arrow over (g)},{right arrow over (f)}) within a two-dimensional coordinate plane, avertical coordinate, corresponding to a horizontal coordinate {rightarrow over (t)}, calculated via linear interpolation is {right arrowover (F)}=interp1[{right arrow over (g)}, {right arrow over (f)}, {rightarrow over (t)}]

Further, the vertical coordinate f_(t), corresponding to the horizontalcoordinate t_(i) is

$f_{t_{i}} = {\frac{g_{p} - g_{q}}{f_{p} - f_{q}} \times {t_{i}.}}$

The t_(i) falls into an interval [g_(p), g_(q)], 0≤q≤i≤p≤N−1, and thef_(p) is a vertical coordinate corresponding to horizontal coordinatesg_(p) and the f_(q) is a vertical coordinate corresponding to horizontalcoordinates g_(q). The correspondence is as shown in FIG. 4 , a curve ofa discrete value of the G(f) is within a coordinate system, and thef_(t), is determined according to the t_(i).

Particularly, piecewise interpolation may be performed to implementconcurrent processing.

At Operation 404: a signal phase vector {right arrow over (ϕ)}=[ϕ₀, ϕ₁,. . . , ϕ_(i)] is obtained by integrating according to the instantaneousfrequency vector, which specifically includes the following operations.

When  i = 0, ϕ₀ = 0.${{{When}\mspace{14mu} i} = 1},{\phi_{1} = {{{{\pi\left( {f_{t_{1}} + f_{t_{0}}} \right)} \times \frac{1}{F_{s}}} + {{\phi_{0}.{When}}\mspace{14mu} i}} = 2}},{\phi_{2} = {{{\pi\left( {f_{t_{2}} + f_{t_{1}}} \right)} \times \frac{1}{F_{s}}} + {\phi_{1}.\ldots}}}$${{{When}\mspace{14mu} 1} \leq i \leq {N - 1}},{\phi_{i} = {{{\pi\left( {f_{t_{i}} + f_{t_{i - 1}}} \right)} \times \frac{1}{F_{s}}} + {\phi_{i - 1}.}}}$

Particularly, it may be appropriate to block for concurrent calculationherein. Instantaneous frequency vectors calculated by blocking in thestep 403 are used to calculate the signal phase vector. For example, twoblocks are provided to calculate {right arrow over (ϕ₁)}=[ϕ₀, ϕ₁, . . ., ϕ_(h)] and {right arrow over (ϕ₂)}=[ϕ_(h+1), ϕ_(h+2), . . . , ϕ_(i)],where 0≤h≤i≤N−1. In a case where ϕ₀=0 and ϕ_(h+1)=0 are set first, the{right arrow over (ϕ)}₁ and the {right arrow over (ϕ)}₂ are calculatedby blocking according to an integration process in the step 404; then, afinal value of the {right arrow over (ϕ)}₁ serves as an initializedvalue of the {right arrow over (ϕ)}₂ to obtain {right arrow over(ϕ₂)}=[ϕ_(h+1), ϕ_(h+2), . . . , ϕ_(i)]+ϕ_(h); and at last, the {rightarrow over (ϕ)}₁ and the {right arrow over (ϕ)}₂ are directly splicedinto an integral phase vector {right arrow over (ϕ)}.

At Operation 405: an NLFM signal is obtained according to the signalphase vector.

In some embodiments, the operation 405 includes an operation that asignal time domain discrete vector is determined according to a signalphase.

In some embodiments, the signal time domain discrete vector {right arrowover (s)} may be denoted as {right arrow over (s)}=[s₀, s₁, . . . ,s_(i)], where, 0≤i≤N−1 and S_(i)=exp(j2πϕ_(i)).

The embodiments of the disclosure provide an apparatus implemented basedon FPGA hardware. As shown in FIG. 5 , the apparatus includes: aninitialization module 51, a group delay vector calculation module 52, aninstantaneous frequency vector calculation module 53, an integrationmodule 54, and a conversion module 55.

The initialization module 51 is configured to initialize a signalparameter as required.

For example, the initialization module 51 is configured to determine arequired signal parameter according to a system parameter such as an SNRand a range resolution, the signal parameter includes a signal bandwidthB, a signal pulse width p and a PSLR; and determine a power spectrumdensity function according to the PSLR. In practice, the signalparameter can be determined according to an instruction parameter sentby a serial port of an upper computer.

The group delay vector calculation module 52 is configured to calculatea group delay vector according to the power spectrum density function.

Specifically, the group delay may be calculated according to the powerspectrum density function as per a following formula.

${G(f)} = {\int_{0}^{f}{\frac{P(f)}{C}df}}$

Where, the f denotes a frequency, and the P(f) denotes a power spectrumdensity function with the frequency as an independent variable. In someembodiments, a window function (for example, a Hamming window, a Kaiserwindow and a Taylor window) may be used as the power spectrum densityfunction. The C is an inverse proportional coefficient,

${C = {\frac{1}{T_{p}}{\int_{B/2}^{{- B}/2}{{P(f)}df}}}}.$

In some embodiments, during hardware implementation, discreteintegration may be used and the group delay is denoted as a group delayvector {right arrow over (g)}=(g₀, g₁, . . . , g_(i)), where, 0≤i≤N−1,the g_(i) denotes a discrete value of G(f) and the N is the number ofsampling points; and N=└F_(s)T_(p)┘, where the └⋅┘ denotes to rounddown.

In some embodiments, a Taylor window function may be selected as thepower spectrum density function, and a corresponding group delayfunction may be denoted as

${{G(f)} = {{\frac{T_{p}}{B}f} + {\sum\limits_{m = 1}^{\overset{\_}{m} - 1}{\frac{F_{m}T_{p}}{\pi m}{\sin\left( {\frac{2\pi m}{B}f} \right)}}}}}.$

Where, the F_(m) is a Taylor window coefficient, the m is the number ofside lobes having a same height, and the m is a calculation processquantity.

The instantaneous frequency vector calculation module 53 is configuredto perform linear interpolation according to the group delay vector todetermine an instantaneous frequency vector. In some embodiments, theinstantaneous frequency vector calculation module 53 may includemultiple blocks.

Specifically, according to signal pulse width, signal bandwidth andsystem sampling rate F_(s), a frequency axial vector {right arrow over(f)} is calculated as{right arrow over (f)}=[f ₀ ,f ₁ , . . . ,f _(i)]

-   -   Where, 0≤i≤N−1,

${f_{i} = {{- \frac{F_{s}}{2}} + {i \times \frac{F_{s}}{N}}}}.$

A time axial vector {right arrow over (t)} is{right arrow over (t)}=[t ₀ ,t ₁ , . . . ,t _(i)]

-   -   Where, 0≤i≤N−1

$t_{i} = {{- \frac{T_{p}}{2}} + {i \times {\frac{1}{F_{s}}.}}}$

The linear interpolation is performed on the group delay vectoraccording to the frequency axial vector {right arrow over (f)}, the timeaxial vector {right arrow over (t)}, to calculate the instantaneousfrequency vector {right arrow over (F)} as{right arrow over (F)}=(f _(t) ₀ ,f _(t) ₁ , . . . ,f _(t) _(i))=interp1[{right arrow over (g)},{right arrow over (f)},{right arrowover (t)}]

The inter1[{right arrow over (g)}, {right arrow over (f)}, {right arrowover (t)}] denotes a one-dimensional linear interpolation operator,particularly, correspondence between horizontal and vertical coordinatescan be determined according to ({right arrow over (g)}, {right arrowover (f)}) within a two-dimensional coordinate plane, a verticalcoordinate corresponding to a horizontal coordinate {right arrow over(t)} calculated via linear interpolation is{right arrow over (F)}=interp1[{right arrow over (g)},{right arrow over(f)},{right arrow over (t)}]

Further, the vertical coordinate f_(t), corresponding to the horizontalcoordinate t_(i) is

$f_{t_{i}} = {\frac{g_{p} - g_{q}}{f_{p} - f_{q}} \times {t_{i}.}}$

The t_(i) falls into an interval [g_(p), g_(q)], 0≤q≤i≤p≤N−1, and thef_(p) is vertical coordinates corresponding to horizontal coordinatesg_(p) and the f_(q) is vertical coordinates corresponding to horizontalcoordinates g_(q).

The instantaneous frequency vector 53 is configured to performintegrating according to the instantaneous frequency vector to obtain asignal phase vector

${\overset{->}{\phi} = \left\lbrack {\phi_{0},\phi_{1},\ldots\mspace{14mu},\phi_{i}} \right\rbrack},{specifically},{{{when}\mspace{14mu} i} = 0},{\phi_{0} = 0.}$${{{when}\mspace{14mu} i} = 1},{\phi_{1} = {{{{\pi\left( {f_{t_{1}} + f_{t_{0}}} \right)} \times \frac{1}{F_{s}}} + {{\phi_{0}.{When}}\mspace{14mu} i}} = 2}},{\phi_{2} = {{{\pi\left( {f_{t_{2}} + f_{t_{1}}} \right)} \times \frac{1}{F_{s}}} + {\phi_{1}.\ldots}}}$${{{When}\mspace{14mu} 1} \leq i \leq {N - 1}},{\phi_{i} = {{{\pi\left( {f_{t_{i}} + f_{t_{i - 1}}} \right)} \times \frac{1}{F_{s}}} + {\phi_{i - 1}.}}}$

Particularly, it may be appropriate to block to calculate the signalphase vector concurrently herein. For example, two blocks are providedto calculate {right arrow over (ϕ)}₁=[ϕ₀, ϕ₁, . . . , ϕ_(h)] and {rightarrow over (ϕ)}₂=[ϕ_(h+1), ϕ_(h+2), . . . , ϕ_(i)], where 0≤h≤i≤N−1. Ina case where ϕ₀=0 and ϕ_(h+1)=0 are set first, the {right arrow over(ϕ)}₁ and the {right arrow over (ϕ)}₂ are calculated by blockingaccording to an integration process in the step 404; then, a final valueof the {right arrow over (ϕ)}₁ serves as an initialized value of the{right arrow over (ϕ)}₂ to obtain {right arrow over (ϕ)}₂=[ϕ_(h+1),ϕ_(h+2), . . . , ϕ_(i)]+ϕ_(h); and at last, the {right arrow over (ϕ)}₁and the {right arrow over (ϕ)}₂ are directly spliced into an integralphase vector {right arrow over (ϕ)}.

The integration module 54 is configured to determine a signal timedomain discrete vector according to the phase vector. In someembodiments, the integration module 54 may include multiple integrationblocks for integral calculation.

In some embodiments, the signal time domain discrete vector {right arrowover (s)} may be denoted as {right arrow over (s)}=[s₀, s₁, . . . ,s_(i)], where, 0≤i≤N−1 and S_(i)=exp(j2πϕ_(i)). The signal time domaindiscrete vector {right arrow over (s)} is a digital signal obtained byFPGA calculation.

The conversion module 55 is configured to generate a discrete signal ofthe signal into a series digital signal, and perform digital-to-analogconversion on the series digital signal to obtain the NLFM signal.

Specifically, the conversion module 55 converts the digital signalgenerated by the discrete value of the signal into an analog signal, toform a corresponding satisfied NLFM signal. In this embodiment, theconversion module 55 may be implemented via a DAC and convert thedigital signal into the analog signal.

The embodiments of the disclosure provide a time sequence structure forgenerating an NLFM signal based on an SAR system. The time sequencestructure is as shown in FIG. 6 , and specifically includes thefollowing operations.

Upon the reception of a serial port instruction transmitted by an uppercomputer at some moment, a signal parameter is initialized first beforethe beginning of next Pulse Repetition Time (PRT). The calculationprocess is started at the end moment of a signal transmitting window forthe next PRT, such that radar waveform data, which is stored in an RAMof an FPGA and will be transmitted in the next PRT, may not be covered;a signal is calculated, and upon the completion of the calculation ofthe corresponding NLFM signal, an output is waited; and the analog NLFMsignal is outputted at a signal transmitting window for a further nextPRT.

According to a calculation time sequence and an FPGA hardwarearchitecture, an initialization parameter is selected as: 80 Mhz ofsignal bandwidth, and 20 us of signal pulse width. A −40 dB Taylorwindow function is selected as a power spectrum density function forinitialization, and a system sampling rate is 90 MHz. An oscilloscope isused to collect a corresponding output analog signal result, with a realpart of a time domain signal as shown in FIG. 7 , and a pulsecompression result as shown in FIG. 8 . A comparison result between ameasured value and a theoretical value for a performance index of theoutput NLFM signal is as shown in table 1.

TABLE 1 Index parameter Theoretical value Measured value PSLR (dB) −39.2−39.1 3-dB main lobe width (sampling point) 1.23 1.23 Integral side loberatio (dB) −29.9 −29.1

As can be seen from the above table, the measured value is close to thetheoretical value, which indicates that the precision of the NLFM signalgenerated by the disclosure in actual applications is close to atheoretical value, and a high-precision technical effect is achieved.

The embodiments of the disclosure further provide a computer readablestorage medium, which is configured to store a calculation programprovided in the above embodiment, to implement the operations of theforegoing method. The computer readable storage medium may be a volatilememory or a nonvolatile memory, and may also include both the volatilememory and the nonvolatile memory. The nonvolatile memory may be a ReadOnly Memory (ROM), a Programmable Read-Only Memory (PROM), an ErasableProgrammable Read-Only Memory (EPROM), an Electrically ErasableProgrammable Read-Only Memory (EEPROM), a Ferromagnetic Random AccessMemory (FRAM), a flash memory, a magnetic surface memory, an opticaldisc or a Compact Disc Read-Only Memory (CD-ROM); and the magneticsurface memory may be a tape memory or a disk memory. The volatilememory may be a Random Access Memory (RAM), and serves as an externalhigh-speed cache. Through illustrative but not restrictive description,many forms of RAMs may be available, for example, a Static Random AccessMemory (SRAM), a Synchronous Static Random Access Memory (SSRAM), aDynamic Random Access Memory (DRAM), a Synchronous Dynamic Random AccessMemory (SDRAM), a Double Data Rate Synchronous Dynamic Random AccessMemory (DDRSDRAM), an Enhanced Synchronous Dynamic Random Access Memory(ESDRAM), a SyncLink Dynamic Random Access Memory (SLDRAM), and a DirectRambus Random Access Memory (DRRAM). The computer readable storagemedium described in this embodiment of the disclosure is intended toinclude but not limited to these and any other suitable types ofmemories, and may also be various devices including any one or anycombination of the above memories, such as a mobile phone, a computer,an intelligent household electrical appliance, and a server.

Each module provided in the embodiments of the application may beimplemented in form of a computer program. The computer program may runin a processor. The program module formed by the computer program may bestored in a memory of a terminal. The computer programs are executed bythe processor to implement the actions of the methods described in theembodiments of the application.

According to the method for generating the NLFM signal in real time andthe apparatus implemented based on the FPGA provided by the embodimentsof the disclosure, a signal parameter is initialized as required; agroup delay vector is calculated according to a set power spectrumdensity function; linear interpolation is performed according to thegroup delay vector to determine an instantaneous frequency vector; asignal phase is obtained by integrating according to the instantaneousfrequency vector; and a discrete value of a time domain signal isdetermined according to the signal phase. Therefore, the complexity ofsignal design is greatly reduced, and the NLFM signal suitable for anFPGA to generate in real time is achieved; and further, compared with anLFM signal, a transmitting power can be saved, an SNR loss is reduced,and thus the system performance is improved.

The method implements a technical effect that the NLFM signal isdirectly generated by means of calculation and digital-to-analogconversion; and by directly converting a calculation result into theNLFM signal, the method optimizes operations of storing, invoking andconverting the calculation result, improves a speed for generating theNLFM signal, and implements a technical effect that the NLFM signal isgenerated in real time.

The above descriptions are only simplified embodiments of the disclosureand are not intended to limit the disclosure. For the person skilled inthe art, the disclosure may have various modifications and changes. Anymodification, equivalent replacement, improvement and the like madewithin a spirit and a principle of the disclosure should be included ina protection scope of the disclosure.

What is claimed is:
 1. A method for generating a Non-Linear FrequencyModulation (NLFM) signal in real time, executed by an apparatus forgenerating the NLFM signal in real time comprising a processor, a memoryand a digital to analog converter, the method comprising: determining asignal parameter of a signal according to a system parameter, whereinthe signal parameter at least comprises: a signal bandwidth, a signalpulse width and a Peak Side Lobe Ratio (PSLR); determining a powerspectrum density function according to the PSLR; calculating the powerspectrum density function to obtain a group delay vector; calculating afrequency axial vector {right arrow over (f)} according to a systemsampling rate F_(s), wherein {right arrow over (f)}=[f₀, f₁, . . . ,f_(i)], where${0 \leq i \leq {N - 1}},{{f_{i} = {{- \frac{F_{s}}{2}} + {i \times \frac{F_{s}}{N}}}};}$calculating a time axial vector {right arrow over (t)} according to thesignal pulse width, wherein {right arrow over (t)}=[t₀, t₁, . . . ,t_(i)], where${0 \leq i \leq {N - 1}},{t_{i} = {{- \frac{T_{p}}{2}} + {i \times \frac{1}{F_{s}}}}},$where T_(p) denotes the signal pulse width; performing linearinterpolation calculation on the group delay vector by using thefrequency axial vector and the time axial vector, to obtain aninstantaneous frequency vector; integrating the instantaneous frequencyvector to obtain a phase vector {right arrow over (ϕ)}, wherein {rightarrow over (ϕ)}=[ϕ₀, ϕ₁, . . . , ϕ_(i)], where when i=0, ϕ₀=0, when${1 \leq i \leq {N - 1}},{{\phi_{i} = {{{\pi\left( {f_{t_{i}} + f_{t_{i - 1}}} \right)} \times \frac{1}{F_{s}}} + \phi_{i - 1}}};}$determining a signal time domain discrete vector according to the phasevector; and generating a digital signal according to the signal timedomain discrete vector, and performing digital-to-analog conversion onthe digital signal via the digital to analog converter to obtain theNLFM signal, wherein calculating the power spectrum density function toobtain the group delay vector comprises: calculating a group delayaccording to ${{G(f)} = {\int_{0}^{f}{\frac{P(f)}{C}df}}},$ where G(f)denotes group delay function, f denotes a frequency, P(f) denotes apower spectrum density function with the frequency as an independentvariable, C is an inverse proportional coefficient,${C = {\frac{1}{T_{p}}{\int_{B/2}^{{- B}/2}{{P(f)}df}}}},$ where Bdenotes the signal bandwidth; using discrete integration to demote thegroup delay as a group delay vector {right arrow over (g)}=(g₀, g₁, . .. , g_(i)), where, 0≤i≤N−1, the g_(i) denotes a discrete value of G(f)and the N is the number of sampling points; and N=└F_(s)T_(p)┘, wherethe └⋅┘ denotes to round down; selecting a Taylor window function as thepower spectrum density function and denoting a corresponding group delayfunction as${{G(f)} = {{\frac{T_{p}}{B}f} + {\sum\limits_{m = 1}^{\overset{\_}{m} - 1}{\frac{F_{m}T_{p}}{\pi m}{\sin\left( {\frac{2\pi m}{B}f} \right)}}}}},$where F_(m) is a Taylor window coefficient, the {right arrow over (m)}is the number of side lobes having a same height, and the m is acalculation process quantity.
 2. The method of claim 1, wherein thedetermining the power spectrum density function according to the PSLRcomprises: obtaining a window function corresponding to the PSLRaccording to the PSLR, and determining the power spectrum densityfunction according to the window function.
 3. The method of claim 1,wherein the calculating the power spectrum density function to obtainthe group delay vector comprises: performing discrete integration on thepower spectrum density function to obtain the group delay vector.
 4. Themethod of claim 3, wherein the performing linear interpolationcalculation on the group delay vector by using the frequency axialvector and the time axial vector to obtain an instantaneous frequencyvector comprises: dividing the group delay vector into n group delaysubvectors, n being a positive integer greater than 1; and performing,by using the frequency axial vector and the time axial vector, thelinear interpolation calculation on the n group delay subvectorsrespectively, to obtain n instantaneous frequency vectors; and theintegrating the instantaneous frequency vector to obtain a phase vectorcomprises: integrating the n instantaneous frequency vectorsrespectively, and splicing integrated results to obtain the phasevector.
 5. An apparatus for generating a Non-Linear Frequency Modulation(NLFM) signal in real time, comprising: a processor; a memory forstoring instructions; and a digital to analog converter, wherein theprocessor is configured to execute the instructions to perform thefollowing operations: determining a signal parameter of a signalaccording to a system parameter, the signal parameter at leastcomprising: a signal bandwidth, a signal pulse width and a Peak SideLobe Ratio (PSLR); determining a power spectrum density functionaccording to the PSLR; calculating the power spectrum density functionto obtain a group delay vector; calculating a frequency axial vector{right arrow over (f)} according to a system sampling rate F_(s),wherein {right arrow over (f)}=[f₀, f₁, . . . , f_(i)], where${0 \leq i \leq {N - 1}},{{f_{i} = {{- \frac{F_{s}}{2}} + {i \times \frac{F_{s}}{N}}}};}$calculating a time axial vector t according to the signal pulse width,wherein {right arrow over (t)}=[t₀, t₁, . . . , t_(i)], where${0 \leq i \leq {N - 1}},{t_{i} = {{- \frac{T_{p}}{2}} + {i \times \frac{1}{F_{s}}}}},$where T_(p) denotes the signal pulse width; performing linearinterpolation calculation on the group delay vector by using thefrequency axial vector and the time axial vector to obtain aninstantaneous frequency vector; integrating the instantaneous frequencyvector to obtain a phase vector {right arrow over (ϕ)}, wherein {rightarrow over (ϕ)}=[ϕ₀, ϕ₁, . . . , ϕ_(i)], where when i=0, ϕ₀=0, when${1 \leq i \leq {N - 1}},{{\phi_{i} = {{{\pi\left( {f_{t_{i}} + f_{t_{i - 1}}} \right)} \times \frac{1}{F_{s}}} + \phi_{i - 1}}};}$determining a signal time domain discrete vector according to the phasevector; and generating a digital signal according to the signal timedomain discrete vector, and performing digital-to-analog conversion onthe digital signal to obtain the NLFM signal via the digital to analogconverter, wherein the processor is further configured to: calculate agroup delay according to ${{G(f)} = {\int_{0}^{f}{\frac{P(f)}{C}df}}},$where G(f) denotes group delay function, f denotes a frequency, P(f)denotes a power spectrum density function with the frequency as anindependent variable, C is an inverse proportional coefficient,${C = {\frac{1}{T_{p}}{\int_{B/2}^{{- B}/2}{{P(f)}df}}}},$ where Bdenotes the signal bandwidth; use discrete integration to demote thegroup delay as a group delay vector {right arrow over (g)}=(g₀, g₁, . .. , g_(i)), where, 0≤i≤N−1, the g_(i) denotes a discrete value of G(f)and the N is the number of sampling points; and N=└F_(s)T_(p)┘, wherethe └⋅┘ denotes to round down; select a Taylor window function as thepower spectrum density function and denote a corresponding group delayfunction as${{G(f)} = {{\frac{T_{p}}{B}f} + {\sum\limits_{m = 1}^{\overset{\_}{m} - 1}{\frac{F_{m}T_{p}}{\pi m}{\sin\left( {\frac{2\pi m}{B}f} \right)}}}}},$where F_(m) is a Taylor window coefficient, the {right arrow over (m)}is the number of side lobes having a same height, and the m is acalculation process quantity.
 6. The apparatus of claim 5, wherein theprocessor is further configured to execute the instructions to: obtain awindow function corresponding to the PSLR according to the PSLR, anddetermine the power spectrum density function according to the windowfunction.
 7. The apparatus of claim 5, wherein the processor is furtherconfigured to execute the instructions to: perform discrete integrationon the power spectrum density function to obtain the group delay vector.8. The apparatus of claim 7, wherein the processor is further configuredto execute the instructions to: divide the group delay vector into ngroup delay subvectors, n is a positive integer greater than 1; perform,by using the frequency axial vector and the time axial vector, thelinear interpolation calculation on the n group delay subvectorsrespectively, to obtain n instantaneous frequency vectors; and integratethe n instantaneous frequency vectors respectively, and spliceintegrated results to obtain the phase vector.
 9. A non-transitorycomputer readable storage medium having computer programs storedthereon, wherein the computer programs, when being executed by anapparatus for generating a Non-Linear Frequency Modulation (NLFM) signalcomprising a processor, a memory and a digital to analog converter,cause the processor to execute the following operations: determining asignal parameter of a signal according to a system parameter, whereinthe signal parameter at least comprises: a signal bandwidth, a signalpulse width and a Peak Side Lobe Ratio (PSLR); determining a powerspectrum density function according to the PSLR; calculating the powerspectrum density function to obtain a group delay vector; calculating afrequency axial vector {right arrow over (f)} according to a systemsampling rate F_(s), wherein {right arrow over (f)}=[f₀, f₁, . . . ,f_(i)], where${0 \leq i \leq {N - 1}},{{f_{i} = {{- \frac{F_{s}}{2}} + {i \times \frac{F_{s}}{N}}}};}$calculating a time axial vector according to the signal pulse width,wherein {right arrow over (t)}=[t₀, t₁, . . . , t_(i)], where${0 \leq i \leq {N - 1}},{t_{i} = {{- \frac{T_{p}}{2}} + {i \times \frac{1}{F_{s}}}}},$where T_(p) denotes the signal pulse width; performing linearinterpolation calculation on the group delay vector by using thefrequency axial vector and the time axial vector to, obtain aninstantaneous frequency vector; integrating the instantaneous frequencyvector to obtain a phase vector {right arrow over (ϕ)}, wherein {rightarrow over (ϕ)}=[ϕ₀, ϕ₁, . . . , ϕ_(i)], where when i=0, ϕ₀=0, when${1 \leq i \leq {N - 1}},{{\phi_{i} = {{{\pi\left( {f_{t_{i}} + f_{t_{i - 1}}} \right)} \times \frac{1}{F_{s}}} + \phi_{i - 1}}};}$determining a signal time domain discrete vector according to the phasevector; and generating a digital signal according to the signal timedomain discrete vector, and performing digital-to-analog conversion onthe digital signal via the digital to analog converter to obtain theNLFM signal, wherein calculating the power spectrum density function toobtain the group delay vector comprises: calculating a group delayaccording to ${{G(f)} = {\int_{0}^{f}{\frac{P(f)}{C}df}}},$ where G(f)denotes group delay function, f denotes a frequency, P(f) denotes apower spectrum density function with the frequency as an independentvariable, C is an inverse proportional coefficient,${C = {\frac{1}{T_{p}}{\int_{B/2}^{{- B}/2}{{P(f)}df}}}},$ where Bdenotes the signal bandwidth; using discrete integration to demote thegroup delay as a group delay vector {right arrow over (g)}=(g₀, g₁, . .. , g_(i)), where, 0≤i≤N−1, the g_(i) denotes a discrete value of G(f)and the N is the number of sampling points; and N=└F_(s)T_(p)┘, wherethe └⋅┘ denotes to round down; selecting a Taylor window function as thepower spectrum density function and denoting a corresponding group delayfunction as${{G(f)} = {{\frac{T_{p}}{B}f} + {\sum\limits_{m = 1}^{\overset{\_}{m} - 1}{\frac{F_{m}T_{p}}{\pi m}{\sin\left( {\frac{2\pi m}{B}f} \right)}}}}},$where F_(m) is a Taylor window coefficient, the {right arrow over (m)}is the number of side lobes having a same height, and the m is acalculation process quantity.
 10. The non-transitory computer readablestorage medium according to clam 9, wherein the computer programs, whenbeing executed by a processor, cause the processor to obtain a windowfunction corresponding to the PSLR according to the PSLR, and determinethe power spectrum density function according to the window function.11. The non-transitory computer readable storage medium according toclam 9, wherein the computer programs, when being executed by aprocessor, cause the processor to perform discrete integration on thepower spectrum density function to obtain the group delay vector. 12.The non-transitory computer readable storage medium according to clam11, wherein the computer programs, when being executed by a processor,cause the processor to: divide the group delay vector into n group delaysubvectors, n is a positive integer greater than 1; perform, by usingthe frequency axial vector and the time axial vector, the linearinterpolation calculation on the n group delay subvectors respectively,to obtain n instantaneous frequency vectors; and integrate the ninstantaneous frequency vectors respectively, and splice integratedresults to obtain the phase vector.